1. Technical Field
This disclosure relates generally to DSL modems and, more particularly, to an adaptively adjustable DSL modem for operation with loops of different lengths.
2. Description of the Background Art
As the Internet continues to become more popular, there is an increasing need for reliably accessing and navigating (“surfing”) the Internet at high speed and at low cost. Providing high speed Internet access permits users to send and receive information over the Internet quickly and reduces the time the user must wait to receive requested information. In addition, there is a continuous pressure to lower the cost of accessing the Internet and Internet-related devices.
One method of accessing the Internet is by using Digital Subscriber Line (DSL) technology. Various versions of DSL exist, such as Asymmetric DSL (ADSL), Symmetric DSL (SDSL), Rate Adaptive DSL (RADSL), Very high speed ADSL (VADSL), Consumer DSL (CDSL), High bit rate DSL (HDSL), etc., and may be collectively referred to as DSL.
ADSL is one version of DSL technology that expands the useable bandwidth of existing copper telephone lines, delivering high-speed data communications at rates up to about 8 Mbps without interrupting normal telephone service, also known as POTS (Plain Old Telephone Service). To achieve this end, ADSL uses frequency-division multiplexing (FDM) technology to carry POTS and ADSL channels all on the same twisted-pair copper telephone line.
A variant of ADSL known as DSL-Lite or G.lite is a DMT-based technology that typically uses about one half of the bandwidth of full-rate ADSL or G.DMT. One advantage of the DSL-Lite-technology is that it has been designed for easy, low-cost deployment and removes the need for a voice-data splitter at every user site.
In one configuration, an ADSL circuit connects an ADSL modem on each end of a twisted copper pair telephone line, creating three information channels. The three channels include a high-speed downstream channel, a medium speed duplex channel, and a POTS (Plain Old Telephone Service) channel. The POTS channel is typically split off from the ADSL modem by filters, thus permitting continuous POTS service, even if the ADSL fails. To create these multiple channels, ADSL modems divide the available bandwidth of a telephone line and typically assign a 4 kHz region for POTS at the DC end of the band.
ADSL is a point-to-point connection technology in that an ADSL termination device, such as an ADSL modem, must be present on each end of the copper circuit. Since ADSL works over copper, it is an appropriate technology for the “local loop,” which comprises the copper circuits running from a central office into virtually every home and business. Conventionally, ADSL service generally requires a local loop length of about 6,000-14,000 feet for optimal service. Indeed, ADSL signals are attenuated as they pass over the loop. Hence, the longer the local loop, the more attenuated the ADSL signal will be upon arriving at the ADSL modem from a central office. While some ADSL service is possible with loop lengths greater than 14,000 feet, it has been found that with loops much longer than about 14,000 feet, the ADSL signal is too attenuated to provide high data transfer rates.
Conversely, for local loop lengths less than about 6,000 feet, the ADSL signal is not sufficiently attenuated for conventional ADSL modems. In particular, conventional ADSL modems typically comprise a fixed high-gain receiver for amplifying and increasing the resolution of the downstream, or incoming, ADSL signal to accommodate ADSL signals that have been attenuated by passing over the local loop. Typically, the first stage receiver of a conventional ADSL modem will have a fixed gain of about 3× to 6×. Consequently, if, due to a relatively short loop length, the ADSL signal is relatively strong when it arrives at the CPE ADSL modem, the strong ADSL signal tends to saturate the first stage receiver and cause distortion, which in turn will cause noise and impair the data connection rate.
Accordingly, a need exists for a DSL modem that may efficiently operate under a variety of loop lengths, including loop lengths of less than about 6,000 feet. An additional need exists to provide a DSL modem that may operate in short loop conditions without having the first stage receiver be saturated by the downstream DSL signal. Moreover, a need exists to provide a system and method by which a DSL modem may accommodate high amplitude downstream DSL signals without significantly impairing the DSL data connection rate.